1. Field of the Invention
The present invention relates to a method and an apparatus for preparation and deaeration of a coating liquid, and particularly relates to a method and an apparatus for removing bubbles in a coating liquid of photothermographic material having a thixotropic nature and having a nature in which components of the coating liquid coagulate when a high level of shear force is applied.
2. Description of the Related Art
When a coating liquid of photographic sensitive material or the like is applied to a base, bubble troubles such as bubble lines and pinholes occur in a coating film on the base if bubbles remain in the coating liquid. Therefore, all bubbles including very small to large bubbles remaining in the coating liquid should be completely removed before the coating liquid is applied to the base.
Conventional deaeration methods for removing bubbles from the coating liquid are as follows.
For one deaeration method, an ultrasonic wave is applied to the coating liquid under pressure. In this deaeration method, principally, bubbles of very small to small sizes in the coating liquid are dissolved in the coating liquid to perform deaeration. This deaeration method has been widely used for deaeration of coating liquids for information recording materials such as photographic sensitive materials and the like of which the performance is influenced by very small bubbles because even very small bubbles existing in the coating liquid can be removed effectively. However, this deaeration method has disadvantages that there may be cases where deaeration is not performed effectively when a large quantity of bubbles exist in the coating liquid and that much time is required for carrying out deaeration, although bubbles of very small to small sizes can effectively be removed.
Other deaeration methods include a centrifugal separation type deaeration method in which the coating liquid is subjected to a centrifugal force to expel bubbles, and a thin film type deaeration method in which the coating liquid is spread into a thin film to expel bubbles. However, this deaeration method is effective for removal of large size bubbles in the coating liquid, but has a disadvantage that it is almost impossible to remove bubbles of very small to small sizes, particularly bubbles of very small sizes.
In addition, as another deaeration method, there is a vacuum deaeration method for removing bubbles and dissolved air in the coating liquid, in which the coating liquid is put in a tank and the pressure in the tank is reduced using a suction pump or the like to perform deaeration.
Conventionally, for reliably removing bubbles of very small to large sizes in the coating liquid, the above described deaeration methods are combined to perform deaeration as disclosed in Japanese Patent Application Publication No. 11-262601.
However, some types of coating liquids have a very strong thixotropic nature (in which a high degree of viscosity is retained under low levels of shearing, and the degree of viscosity gradually decreases as the level of shearing is increased) compared with the photographic sensitive material described above, and have a difficult nature in which components of the coating liquid coagulate when a high level of shear force is applied to the coating liquid, like the coating liquid of photothermographic material for example.
Therefore, in the case of the coating liquid of photothermographic material, the viscosity under low levels of shearing is so high that the speed at which bubbles move in the coating liquid is very low and thus a considerable amount of time is required for performing deaeration in ordinary vacuum deaeration. Nevertheless, if a high level of shear force is applied to the coating liquid to reduce the viscosity before performing deaeration, components of the coating liquid coagulate as described above, and consequently the quality of the coating liquid is adversely affected.
Also, the deaeration method using a centrifugal force cannot be applied to the photothermographic material because eventually, a high level of shearing force is applied to the coating liquid, and therefore components of the coating liquid coagulate. On the other hand, the deaeration method in which the coating liquid is spread into a thin film to expel bubbles has a disadvantage that the treatment efficiency is so low that a large amount of coating liquid cannot be treated although a high level of shear force is not applied to the coating liquid.
From the background described above, in the case where the centrifugal separation type deaeration step is included in the technique disclosed in Japanese Patent Application Publication No. 11-262601, the method cannot be applied to the coating liquid of photothermographic material although it can be applied to the coating liquid of photographic sensitive material. Also, in the case where the thin film type deaeration step is included in the technique disclosed in Japanese Patent Application Publication No. 11-262601, a large amount of coating liquid cannot be treated because deaeration performance is inevitably reduced in the thin film type deaeration step. Accordingly, a method and an apparatus capable of reliably removing bubbles of very small to large sizes irrespective of the nature of the coating liquid, and treating a large amount of coating liquid, and eliminating the possibility that the quality of the deaerated coating liquid is adversely affected have been required.